A variety of imaging techniques are available for diagnostic purposes. Such imaging techniques are generally non invasive and include magnetic resonance imaging (MRI) and ultrasound. Contrast agents are used in imaging to increase the signal difference between the area of interest and background. Such agents can be divided into two general categories, those passive agents which non-specifically enhance the signal that is produced and targeted contrast agents which are chemically modified in order to localise to a specific cell type or tissue through an active mechanism.
Targeted contrast agents that can be localised to a particular cell or tissue type can potentially allow the non-invasive visualisation of a variety of disease states. Such targeted agents need to overcome the difficulties of high background signal in order to provide a clear picture for the clinician. There is a need for improved contrast agents that can be targeted to cell surface receptors to enhance imaging techniques.